The present invention describes a method for control of synchronous electrical motors with application to Stepper Motors and Brushless Direct Current Motors for which the currents in their stator windings are regulated with Pulse Width Modulation. The invention enables continuous in time sensorless determination of the load angle and the angular speed of the motor and consequently it enables sensorless closed-loop motor control.
A method for stepper motor control is known where the back-electromotive force voltages induced in the stator windings of the motor are measured only in the time intervals when there is no current flowing through them. During every full electrical period of the currents in the stator windings of a two-phase bipolar stepper motor there are four time intervals when there is no current in one of the two windings. The voltages on the stator windings' terminals during these time intervals are measured and the measured values are compared with a given set value. If these voltages are greater than the set value it is assumed that the rotor of the motor is moving, if these voltages are smaller than the set value it is assumed that the rotor is stalled. This method relies on measuring the back-electromotive force voltages induced by the moving rotor in the stator windings where these voltages can be measured only in the intervals of time when there is no current through the windings.
One of the shortcomings of the above method is that the monitoring of the rotor rotation can be done only in the time intervals when there is no current through one of the stator windings. When there is current flowing through the windings it is not possible to monitor the rotor rotation. Also from the measured values of the back-electromotive force voltages it is not possible that the load angle of the motor is determined, which does not allow controlling the motor speed depending on the motor loading.
A method for control of brushless direct current motors is also known where the currents in the stator windings of the motor are controlled depending on the rotor position, which in turn is determined with Hall sensors. Depending on the position of the rotor magnetic field, the Hall sensors generate logical signals zero or one at their outputs. Using these logical signals the rotor position is determined and the currents in the stator windings are controlled in order to maintain a certain load angle of the motor.
A shortcoming of this method is the necessity of using Hall sensors or other means for rotor position monitoring which leads to higher complexity and higher price.